We're all familiar with the power that hormones have over behavior and mood, but did you know that sex and stress hormones can affect how we recall past events? Now, a recently published study has shed light on the memory-altering effects of one of the most widely used hormone-regulating drugs in the world: birth control.

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According to neurobiologists at UC Irvine, hormonal contraceptives like the pill have a strong and measurable influence on the way women remember emotionally charged events.

The researchers arrived at this conclusion after an experiment involving 72 women who were either naturally cycling or on the pill. Every woman in the study viewed either a brief, narrated story containing "emotionally arousing" elements or one containing only "neutral" elements.

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The emotionally arousing narrative told the story of a woman, her son, and a car accident, wherein the car hits the boy and critically injures him. In the emotionally neutral narrative, the car merely hits a curb. Both stories were accompanied by 11 photographs depicting the woman, her son, and the accident (although the 11 photographs were the same across both stories.)

A week later, the tested women were administered a surprise free recall test. The researchers found that women on the pill experienced enhanced memory of the overall events of the story — or what the researchers refer to as the "gist" of the narrative — compared to neutral story conditions. (That is to say the women could accurately recall the main steps in the traumatic event — that the boy had been hit by a runaway car and taken to a nearby hospital, for example.)

On the other hand, naturally cycling women demonstrated what the researchers describe as "enhanced memory of story details" (that there had been a fire hydrant next to the crashed car, or that the hospital had been light brown in color, for example), but not of overall events.

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"What's most exciting about this study is that it shows the use of hormonal contraception alters memory," said Shawn Nielsen, one of the researchers involved in the study. "There are only a handful of studies examining the cognitive effects of the pill, and more than 100 million women use it worldwide."

It bears mentioning that the researchers did not find evidence that the pill damages memory in any way; Nielson emphasizes that "it's a change in the type of information [the women] remember, not a deficit."